A great question.
Answer in nutshell
Questions and answers fully written in Tamil are allowed and should be allowed at all cost. A English translation can be added by the community members to help beginers.
Questions that are purely in English are also welcomed.
There may be some native users of Tamil Language who may not be comfortable with English. We cannot force them to use English. This site is afterall about Tamil language. Users will be encouraged to post on both languages or add a translated english text to the post. This will help the begineers.
We should take the examples from the already existing language based sites like Japanese, Chinese, French, etc...
From French meta site,
The primary language of the site should be French, and most questions
and answers on the site should be in French, since the experts on the
topic of the site (the French language) are by definition fluent in
French and may not be able to understand or write in English. It is
better for learners to post in halting but comprehensible French (and
have their question improved) than to post in English only to have
their question misunderstood. Furthermore a large proportion of
questions that are not in French would drive away many native French
speakers. Again, we don't want to drive the experts away.
From Japanese Meta site,
I'm all for encouraging more native speakers to use the site as well,
most of the time native speakers are the best ones to answer questions
about the finer nuances of the use of various terms or even the
regional differences between how things are said. That said through, I
do think that we should encourage people to translate questions and
answers that are purely in Japanese (to a reasonable extent) so they
are approachable by beginners.
Almost all the other language sites say the same thing. I don't want to quote them all here.
I believe that posts fully written in Tamil language must be allowed.